This invention relates to a taper cutting control method and system in a wire-cut electric discharge machine. More particularly, the invention relates to a taper cutting control method and system in a wire-cut electric discharge machine which enables the cutting speed at a predetermined taper-cut surface (a surface which is taper-cut by the electric machining of a workpiece through the intervention of a wire electrode) to be brought into coincidence with a commanded speed, and which, in the taper cutting operation, allows the movement of the upper and lower guides for guiding the wire electrode to be started simultaneously and ended simultaneously.
As is well-known in the art, a wire-cut electric discharge machine has a wire stretched between an upper guide and a lower guide and machines a workpiece by producing an electrical discharge between the wire and the workpiece. The workpiece, fixed on a table, is transported in X and Y directions along a machining contour in response to commands from a numerical control device.
Specifically, as shown in the simplified illustrative view of FIG. 1, which illustrates a well-known wire-cut electric discharge machine, a wire WR is taken up by a reel RL.sub.2 and supplied with a voltage by a contacting electrode, not shown, while being payed out by a reel RL.sub.1 and tensioned between the lower guide DG and upper guide UG. An electrical discharge is produced between the wire WR and the workpiece WK to cut the workpiece. The workpiece WK, meanwhile, since it is fixed on an X-Y table TB which is transported in the X and Y directions by motors MX, MY, can be cut into any profile by moving the table in the X and Y directions. Further, the arrangement is such that the upper guide UG is mounted on a moving mechanism that is transported in the X and Y directions by motors MU, MV, so that the upper guide UG also can be transported in the X and Y directions.
The above-mentioned moving mechanism, reels RL.sub.1, RL.sub.2 and lower guide DG are mounted on a column CM.
A numerical control device NC reads the contents of a command tape TP, executes distribution processing along each axis by means of distributing circuits DS in accordance with the commands, and drives motors MX, MY, MU, MV for each axis by means of driver circuits SVX, SVY, SVU, SVV corresponding to the respective axes to actuate the table TB and moving mechanism along a plane parallel to an X-Y plane to cut the workpiece WK into the desired profile.
With a four-axis control wire-cut electric discharge machine, the upper and lower surfaces of the workpiece are machined into profiles which are identical when the stretched wire is held normal to the table TB (workpiece WK). If the above-mentioned moving mechanism displaces the upper guide UG in the X and Y directions (referred to as the U- and V- axes) to incline the wire WR with respect to the workpiece WK such as by displacing the upper guide in a direction at right angles to the direction of workpiece movement, then the upper and lower surfaces of the workpiece WK will not be machined to the same profile. Instead, the surface machined by the wire will be inclined. This is so-called taper cutting.
FIG. 2 is an illustrative view of such taper cutting, in which a wire WR is stretched between an upper guide UG and a lower guide DG at a predetermined angle of inclination with respect to a workpiece WK. If we take the lower surface PL of the workpiece WK as the commanded program profile (the upper surface QU of the workpiece WK may also serve as a programmed profile), and if we let .alpha. denote the taper angle, H the vertical distance between a plane in which the upper guide UG is movable and a plane in which the lower guide DG is movable, and h the vertical distance between the plane in which the lower guide DG is movable and and the lower surface of the workpiece WK, then the offset d.sub.1 of the lower guide DG and the offset d.sub.2 of the upper guide UG with respect to the lower surface PL of the workpiece, may be expressed: ##EQU1## Note that d is the width of the cut.
Thus, if movement of the upper guide UG from which the wire WR is stretched is so controlled in relation to workpiece movement that the offsets d.sub.1, d.sub.2 remain constant, then taper cutting at the taper angle .alpha. can be carried out, as shown in FIG. 3. The dashed line and one-dot chain line in the FIG. 3 indicate the paths of the upper and lower guides UG, DG, respectively. Designated in FIG. 10 at TUG is the path of movement of the upper guide, and TDG the path of movement of the lower guide.
In such taper cutting with a wire-cut electric discharge machine, cutting is performed in accordance with commands generally relating to programmed path data for the upper or lower surface of the workpiece (such as data for identifying end point coordinates, straight lines and circles), feed speed on the programmed path, taper angle, the distances H, h, etc. These commands are applied from a numerical control system to the wire-cut electric discharge machine.
With taper cutting, however, the distance from the cutting starting point to the end point generally is different along the upper and lower guide paths and on the cut surface. To cut a highly accurate tapered surface under such conditions, it is required that the cutting speed on the cut surface be brought into agreement with the commanded speed, and that the movement of the upper and lower guides start simultaneously and end simultaneously.
Conventionally, however, taper cutting is carried out by satisfying such requirements through an extremely complicated method.
Another numerical control system for controlling a travelling-wire electronic discharge machine is disclosed in U.S. Pat. No. 4,355,223. In this system, the pulses from the numerical controller are transmitted to pulse motors for driving the workpiece table along X and Y axes for generating the shape to be machined in the workpiece. A taper is applied to the cut by mounting one of the electrode wire guides on a subsidiary cross table capable of x and y movements through respective pulse distributors and sign-changing inverters from the numerical controller with respective pulse motors so that the cut on one side of the workpiece represents the sum of increments from both the X-Y and x-y displacements, while the cut on the opposite side represents substantially only the increments of the x-y displacements.
In this system, X-axis feed pulses and Y-axis feed pulses issued from the X- and Y-axis pulse distributors (interpolators) are applied to X- and Y-axis servomotors, respectively, to drive a main cross table which supports the workpiece thereon. At the same time, the products of the X- and Y-axis feed pulses and the ratio of the amount of feed TM by the main cross table to the amount of feed TS by the subsidiary cross table which moves one end of the wire electrode in x-y directions are used as feed pulses along x and y axes. Therefore, for linear interpolation, upper and lower machining lines on the workpiece are parallel as viewed in the direction of the z-axis.
This means that it is impossible to machine the workpiece along two machining lines which are not parallel to each other, i.e., along a twisted tapered surface.
For circular interpolation, upper and lower arcuate machining lines on the workpiece as viewed in the direction of the Z-axis must be concentric, and their angles of arc must be identical to each other.
In machining the workpiece along a path as shown in FIG. 7 of U.S. Pat. No. 4,355,223, when the guide on the subsidiary cross table reaches a point P along a line L1, the workpiece on the main cross table also reaches a point P' at the same time. After only the main cross table has moved from the point P' to a point P", do the two cross tables proceed from the points P, P" to points Q, Q", respectively. After having reached the points Q, Q", the main cross table remains at the point Q", whereas the movable guide moves from the point Q to a point Q', after which both tables leave the points Q', Q" for the direction of L2.